10.2 Fertilisers Revision Notes
1. Overview
Fertilisers are chemical substances added to soil to replace essential mineral ions that are absorbed by plants as they grow. By providing these nutrients, fertilisers ensure healthy plant development, increase the speed of growth, and improve overall crop yields to support the global food supply.
Key Definitions
- Fertiliser: A natural or synthetic substance applied to soil or plant tissues to supply one or more nutrients essential to the growth of plants.
- NPK Fertiliser: A multi-nutrient fertiliser containing the three primary elements required for plant growth: Nitrogen (N), Phosphorus (P), and Potassium (K).
- Yield: The mass of food produced from a specific area of land.
Core Content
Essential Elements for Plant Growth
Plants require several elements to grow, but three are particularly important and are often depleted in agricultural soils:
- Nitrogen (N):
- Role: Essential for making chlorophyll and proteins. It promotes healthy leaves and rapid stem growth.
- Source: Provided in the form of ammonium salts and nitrates.
- Phosphorus (P):
- Role: Essential for energy transfer and healthy root development.
- Source: Provided in the form of phosphate compounds.
- Potassium (K):
- Role: Essential for the production of flowers and fruits and helps the plant resist diseases.
- Source: Provided in the form of potassium salts (e.g., potassium chloride or potassium sulfate).
Ammonium Salts and Nitrates
Most nitrogen-based fertilisers are produced using ammonia (NH₃). Ammonia is an alkali that reacts with acids to form soluble ammonium salts. These salts are easily absorbed by plant roots when dissolved in soil water.
- Ammonium Nitrate: One of the most common nitrogen fertilisers.
- Word Equation: Ammonia + Nitric acid → Ammonium nitrate
- Symbol Equation: NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq)
- Ammonium Sulfate:
- Word Equation: Ammonia + Sulfuric acid → Ammonium sulfate
- Symbol Equation: 2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)
- Ammonium Phosphate:
- Word Equation: Ammonia + Phosphoric acid → Ammonium phosphate
- Symbol Equation: 3NH₃(aq) + H₃PO₄(aq) → (NH₄)₃PO₄(aq)
NPK Fertilisers
Commercially available fertilisers are often "NPK" fertilisers. These are mixtures that provide all three essential elements in a single application. The ratios of N, P, and K can be adjusted depending on the specific needs of the crop or the soil type.
Extended Content (Extended Only)
There are no specific Supplement-only learning objectives for this sub-topic in the current syllabus.
Key Equations
| Reaction | Word Equation | Balanced Symbol Equation |
|---|---|---|
| Formation of Ammonium Nitrate | Ammonia + Nitric acid → Ammonium nitrate | NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq) |
| Formation of Ammonium Sulfate | Ammonia + Sulfuric acid → Ammonium sulfate | 2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq) |
| Formation of Ammonium Phosphate | Ammonia + Phosphoric acid → Ammonium phosphate | 3NH₃(aq) + H₃PO₄(aq) → (NH₄)₃PO₄(aq) |
Symbols and Units:
- NH₃: Ammonia gas (or aqueous solution)
- NH₄⁺: Ammonium ion
- NO₃⁻: Nitrate ion
- (aq): Aqueous (dissolved in water)
Common Mistakes to Avoid
- ❌ Wrong: Thinking that Potassium's chemical symbol is P.
- ✓ Right: Remember that P is for Phosphorus and K is for Potassium.
- ❌ Wrong: Writing the formula for the ammonium ion as NH₃.
- ✓ Right: Ammonia is NH₃; the ammonium ion (found in salts) is NH₄⁺.
- ❌ Wrong: Forgetting to balance the charges in ammonium sulfate.
- ✓ Right: Because the sulfate ion is SO₄²⁻ and the ammonium ion is NH₄⁺, the formula must be (NH₄)₂SO₄.
Exam Tips
- Command Words: If asked to "State" the elements in NPK, simply list Nitrogen, Phosphorus, and Potassium. If asked to "Describe" the use of fertilisers, mention that they replace lost minerals to increase crop yield.
- Calculation Questions: You may be asked to calculate the percentage by mass of nitrogen in a fertiliser. Use the formula: $$% \text{ by mass} = \frac{(\text{Number of atoms of element} \times A_r)}{\text{Total } M_r \text{ of compound}} \times 100$$
- Naming Salts: Always look at the acid used. Nitric acid makes nitrates, sulfuric acid makes sulfates, and phosphoric acid makes phosphates.
- Solubility: Remember that all ammonium salts and all nitrates are soluble in water, which is why they are effective fertilisers—plants can only take up nutrients that are dissolved in water.